WO1995002018A1 - Coating powder, method of coating metal pipes with the powder, and use of the coating powder for the single-coat coating of the outside surfaces of metal pipes - Google Patents

Abstract

The invention concerns coating powders containing epoxy resins, phenolic cross-linking agents, catalysts, fillers and, optionally, auxiliaries and additives. The coating powders are characterized in that crystalline silicilic acids modified to include glycidyl functional groups are used as fillers. The invention also concerns a method of coating the outside surfaces of metal pipes with the coating powders described and the use of the coating powders for the single-coat coating of the outside surfaces of metal pipes.

Description

Powder coating, process for the external coating of metal pipes and use of the powder coating for single-layer external coating of metal pipes

The present invention relates to powder coatings, containing epoxy resins, phenolic crosslinking agents, catalysts, fillers and optionally auxiliaries and additives. The invention further relates to methods for the external coating of metal pipes with these powder coatings and the use of the powder coatings for single-layer external coating of metal pipes.

Today, metal pipes are usually coated with reactive powder coatings to protect them against corrosion. It is known to use powder coatings based on epoxy resins and suitable crosslinking agents for this purpose. The powder coatings suitable for this single-layer coating method for metal pipes must meet high requirements with regard to corrosion protection, hot water loading and with regard to cathodic delamination. Powder coatings suitable for outer tube coating are known, for example, from EP-B-104 719 and US Pat. No. 4,122,060. The powder coatings known from EP-B-104 719 are epoxy resins which are crosslinked, for example, with phenolic hardeners. The powder coatings described also contain catalysts and calcium oxide as a filler.

US Pat. No. 4,122,060 describes powder coatings based on epoxy resins, hardeners, fillers and catalysts, amorphous silicas preferably being used as the filler. Furthermore, it is known to use field and heavy spades or even precipitated barium sulfate as fillers in epoxy powder coatings for tube coating.

The previously known powder coatings based on epoxy resins, suitable crosslinking agents, catalysts and fillers, such as Amorphous silicas, field and heavy spades and precipitated barium sulfates have the disadvantage that they have poor properties with regard to the resistance to hot water and the resistance to cathodic delamination (DIN 30671).

The present invention was therefore based on the object of eliminating the disadvantages of the prior art, i.e. Powder coatings based on epoxy resins, suitable crosslinking agents, e.g. to provide phenolic hardeners, fillers and catalysts which are suitable for the outer coating of metal tubes. The pipes coated with the powder coatings should have good corrosion protection, improved resistance to hot water and improved resistance to cathodic delamination. Furthermore, the coatings obtained should have a high degree of flexibility.

The object on which the invention is based is achieved by powder coating materials containing epoxy resins, phenolic crosslinking agents, catalysts, fillers and optionally auxiliaries and additives. The powder coatings are characterized in that crystalline silica modifications functionalized with glycidyl groups are used as fillers.

Suitable epoxy resins are all solid epoxy resins with epoxy equivalent weights between about 400 and 3,000. These are mainly epoxy resins based on bisphenol A and bisphenol F. Epoxidized novolak resins are particularly preferred.

Mixtures of bisphenol-A or bisphenol-F resins and novolak resins are also suitable. The epoxy resins based on bisphenol A and bisphenol F generally have a functionality <2, the epoxidized novolak resins a functionality> 2. Epoxidized novolak resins with an average functionality in the range from 2.4 to 2.8 and with an epoxy equivalent weight in the range from 600 to 850 are particularly preferably used in the powder coatings according to the invention. In the epoxidized novolak resins, the phenolic hydroxyl groups are etherified with alkyl, aryl or similar groups. By reacting the phenolic hydroxyl groups with epichlorohydrin, epoxy groups are incorporated into the molecule. Starting from novolaks, the so-called epoxy novolak is formed. The epoxidized novolaks are structurally related to bisphenol A resins. Epoxidized novolak resins can be made by epoxidizing novolaks, e.g. consist of 3 to 4 phenol cores, which are connected to each other via methylene bridges. Alkyl-substituted phenols which are reacted with formaldehyde can also be used as novolak resins.

Suitable epoxy resins are, for example, the products commercially available under the following names: Epikote 154, 1001, 1002, 1055, 1004, 1007, 1009, 2014, 3003-4F-10 from Shell-Chemie, XZ 86795 and DER 664, 667, 669, 662, 642U and 672U from Dow and Araldit XB 4393 , XB 4412, GT 7072, GT 7203, GT 7004, GT 7304, GT 7097, GT 7220 and GT 7255 from Ciba Geigy.

For curing the epoxy resins, the powder coating according to the invention contains phenolic crosslinking agents. Any phenolic resin can be used, for example, as long as it has the methylol functionality required for reactivity. Preferred phenolic resins are reaction products of phenol, substituted phenols and bisphenol A with formaldehyde produced under alkaline conditions. Under such conditions, the methylol group is linked either ortho or para to the aromatic ring. According to the present invention, bisphenol-A or bisphenol-F resins containing hydroxyl groups and having a hydroxy equivalent weight in the range from 180 to 600, particularly preferably in the range from 180 to 300, are particularly preferably used as phenolic crosslinking agents. Such phenolic crosslinking agents are prepared by reacting bisphenol-A or bisphenol-F with components containing glycidyl groups, such as e.g. the diglycidyl ether of bisphenol-A. Such phenolic crosslinking agents are available, for example, under the trade names DEH 81, DEH 82 and DEH 87 from Dow, DX 171 from Shell-Chemie and XB 3082 from Ciba Geigy.

The epoxy resins and the phenolic crosslinking agents are used in such a ratio that the number of epoxy groups to the number of phenolic OH groups is approximately 1: 1. The powder coatings according to the invention contain one or more suitable catalysts for epoxy resin curing. Suitable catalysts are phosphonium salts of organic or inorganic acids, idazole and imidazole derivatives, quaternary ammonium compounds and amines. The catalysts are generally used in proportions of 0.001% by weight to about 10% by weight, based on the total weight of the epoxy resin and the phenolic crosslinking agent.

Examples of suitable phosphonium salt catalysts are ethyltriphenylphosphonium iodide, ethyltriphenylphosphonium chloride, ethyltriphenylphosphoniumthio- cyanate, ethyltriphenylphosphonium acetate-acetic acid complex, tetrabutylphosphonium iodide, tetrabutylphosphonium acylium bromide-aconium-bromobium-acetic acid. These and other suitable phosphonium catalysts are e.g. described in U.S. Patent 3,477,990 and U.S. Patent 3,341,580.

Suitable imidazole catalysts are, for example, 2-styrylimidazole, 1-benzyl-2-methylimidazole, 2-methylimidazole and 2-butylimidazole. These and other imidazole catalysts are e.g. described in Belgian Patent No. 756,693.

Some commercially available phenolic crosslinking agents already contain catalysts for epoxy resin crosslinking.

The powder coating materials of the invention are characterized in that they contain glycidyl group-functionalized crystalline silica modifications as fillers. These fillers are usually in a proportion of 10 to 50 wt .-%, based on the

Total weight of the powder coating used. In some Cases of filler contents of more than 50% by weight are also possible.

The crystalline silica modifications include quartz, cristobalite, tridymite, keatite, stishovite, melanophlogite, coesite and fibrous silica. The crystalline silica modifications are functionalized with glycidyl groups, the functionalization of the glycidyl groups being achieved by a surface treatment. These are, for example, silica modifications based on quartz, cristobalite and fused silica, which are produced by treating the crystalline silica modifications with epoxysilanes. The glycidyl group-functionalized silica modifications are available on the market, for example, under the names Silbonß ^y 600 EST and Silbono® 6000 EST (manufacturer: Quarzwerke GmbH).

The powder lacquers according to the invention advantageously contain 10 to 40% by weight, based on the total weight of the powder lacquer, of glycidyl group-functionalized crystalline silica modifications.

The powder coatings can also contain other inorganic fillers, for example titanium dioxide, barium sulfate and fillers based on silicate, such as e.g. Talc, kaolin, magnesium, aluminum silicates, mica and the like are included. In addition, the powder coatings may also contain auxiliaries and additives. Examples of these are leveling agents, pouring aids and deaerating agents, such as, for example, benzoin.

The powder coatings are produced by known methods (cf., for example, product information from BASF Lacke + Farben AG, "Powder Coatings", 1990) by means of homogeneous sieren and dispersing, for example by means of an extruder, screw kneader, etc. After the powder coatings have been produced, they are adjusted to the desired particle size distribution by grinding and, if necessary, sifting and sieving.

The powder coatings are applied electrostatically or tribo-electrostatically to the previously heated metal pipe surface.

The invention therefore also relates to a method for the external coating of metal pipes with the powder coatings described above. In this process, the metal pipe surface is first cleaned of rust, grease, oil, dust, etc. If necessary, a chemical pretreatment (chromating and / or phosphating) is carried out. The cleaned metal pipes are then heated to a coating temperature of approximately 170 to 250 ° C. by inductive heating or in a gas oven. The powder coatings according to the invention are applied electrostatically or by means of friction charging to the hot metal tube surface. Usual application thicknesses of the powder coating are in the range from 100 to 1000 μm, preferably in the range from 300 to 500 μm.

The powder coatings harden within a few minutes.

The present invention further relates to the use of the powder coatings described above for the external coating of metal pipes.

The coated metal tubes obtainable by the process according to the invention have excellent properties. So there is no detachment of the powder lacquer from the surface. The hot water resistance of the powder coating coatings obtained is excellent, and the results of the CD test, which tests the resistance of the powder coating to cathodic delamination in accordance with DIN 30671, are excellent.

The invention is explained in more detail below with the aid of examples. Parts mean parts by weight, unless otherwise stated.

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OI o Ot σ Ul 01

Powder coatings of the following composition are produced:

Example 1 Comparative Comparative Example 1 Example 2

Pigment (parts by weight) 2.0 2.0 2.0

Blanc finxe-N (parts by weight) - 19.0 -

Minex 4 (wt.) - - 19.0

Silbond 6000 EST 19.0 - -

(glycidyl group-functionalized crystalline silicic acid) (parts by weight)

Additives (wt.) 1.66 1.66 1.66

Catalyst, EPON P104 from Shell 0.5 0.5 0.5

(Wt.) Phenolic crosslinking agent, 13.8 13.8 13.8

XD 8062 from Dow (wt.%)

Epoxy resin, trade name 31.0 31.0 31.0

THE 664U from Dow

Novolak resin, trade name 32.0 32.0 32.0

THE 672U from Dow

Aerosil R 972 from Degussa as 0.04 0.04 0.04

Trickle aid

The powder coatings of Example 1 and Comparative Examples 1 and 2 are processed into powder coatings with a commercial particle size distribution.

The powder coatings produced are used in the single-layer process for the external coating of metal pipes. For this purpose, pipes with a diameter of 300 mm and a wall thickness of 12 mm are blasted in a blasting system on cleanliness SA 3. The roughness should be approx. 50 μm. The tubes are then heated to 230 + 5 ° C with an induction coil. The powder coatings of Example 1 and Comparative Examples 1 and 2 are applied and cured electrostatically with a layer thickness of approximately 100 μm.

The test results are summarized below.

Example 1 Comparative Comparative Example 1 Example 2

Gel time ¹⁾ | 59 sec. 56 sec. 58 sec. 180 ° C

Cupping and ²⁾ 1 8.0 + 7.9 + 8.3 + bend (table edge) on 0.5 mm steel sheet, 10 '180 ° C

Layer thickness (μm) | 60- 70 80 - 90 80 - 90 gloss 60 ° | 87 E 90 E 75 E course 10'180 ° C | moderately moderately moderately

Bend ³ • ^* RT | 32 mm 12 mm 15 mm

| 32 mm 10 mm 13 mm approx. 0 ° | 18 mm 6 mm 11 mm

| 18 mm 7 mm 10 mm 1) determined with Coesfeld gel time device 2) 0 = cracked; * = Hairline cracks; + = OK 3) blasted on a 5 mm board with Wheelabrator GH 40, SA 2.5 240 ° C pre-annealed, coated, baked for 120 seconds at 240 ° C and immediately cooled in water: layer thickness 400 - 500 μm. The higher the bending values, the higher the flexibility.

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Ul o Ul O Ul Ul

Impact on 10 mm plate blasted with Wheelabrator GH 40, SA 2.5 240 "C, pre-annealed, coated, baked for 120 seconds at 240 ° C and immediately cooled in water:

Example 1 Comparative Comparative Example 1 Example 2

Layer thickness imp. ¹ ) Layer thickness imp. ¹⁾ Layer thickness imp. Μm kg * cm μm kg * cm μm kg * cm

Item 1 550 (60) 670 (50) 540 (50)

Item 2 570 55 660 (45) 560 40

Item 3 570 (60) 660 (40) 560 45

Point 4 610 55 670 35 570 (50)

Item 5 550 (55) 650 (40) 540 45

Point 6 600 55 640 35 610 50

Item 7 550 (60) 600 (35) 520 50

Point 8 600 55 600 35 540 (55)

Impact values without () = OK, there is no destruction of the film. Impact values with () = not ok, i.e. H. the film is destroyed

^ ': The value given is the product of the weight of the falling body (kg) and the falling height (cm).

Water storage at 80 ° C in tap water to 5 mm

Blasted board with Wheelabrator GH 40, SA 2.5, dipped in 10 vol% in Basomat PT, pre-annealed at 240 ° C, coated, baked for 120 seconds at 240 ° C and immediately cooled in water. Layer thickness: 400-500 μm

Example 1 Comparative | example 1 example 2 |

Hot cold hot | Cold hot | Cold |

unloaded + 1 * 1 * 1

168 hours + + - | * _ | - |

336 hours + + - | * 1 1

504 hours + + - | * - | - |

672 hours + + - | - - | - |

840 hours + / * + * / _ | * / _ 1 - 1

1008 hours + / * + / * - | - _ | - |

++ = very good, + = giit, * = moderate, - = bad,

- = very bad

CD test on 10 mm plates blasted with Wheelabrator GH 40, SA 2.5, immersed in 10 vol% Basomat PT, preheated to 240 ° C., coated, baked for 120 seconds at 240 ° C. and immediately cooled in water . Layer thickness: 400 - 500 μm Unterwande¬ example 1 comparative comparison according to example 1 example 2

30 days RT 0 - 1 mm X X

2 days 65 ° C 0 mm X X

14 days 65 ° C 1 mm X X

x - poor liability, therefore no infiltration was found.

Water storage free film at 80 "C in tap water: layer thickness: 400 - 500 μm

Wasserauf¬ Example 1 | Comparative comparison according to example 1 example 2

240 hours 5.20% | 8.14% 6.01%

504 hours 5.86% | 9.89% 8.82%

744 hours 6.21% | 11.06% 9.28%

1008 hours 7.05% | 12.22% 10.58%

Claims

claims Powder coating containing epoxy resins, phenolic crosslinking agents, catalysts, fillers and, if appropriate, auxiliaries and additives, characterized in that crystalline silicic acid modifications functionalized with glycidyl groups are used as fillers. 2. Powder coating according to claim 1, characterized in that epoxidized novolak resins with an average functionality in the range from 2.4 to 2.8 and with an epoxy equivalent weight in the range from 600 to 850 are used as epoxy resin. 3. Powder coating according to Claim 1 or 2, characterized in that bisphenol-A or bisphenol-F resins containing hydroxyl groups and having a hydroxy equivalent weight in the range from 180 to 600 are used as phenolic crosslinking agents. 4. Powder coating according to claim 1 to 3, characterized gekenn¬ characterized in that it contains 10 to 40 wt .-%, based on the total weight of the powder coating, glycidyl group-functionalized crystalline silica modifications. 5. Process for the single-layer outer coating of metal pipes with a powder coating, containing Epoxy resins, phenolic crosslinking agents, catalysts and fillers, characterized in that the powder coating according to claims 1 to 4 is used. Use of the powder coating according to Claims 1 to 4 for the single-layer outer coating of metal tubes.